feat: add HOP --FF variant

[ax0]

Description

This PR adds a channel-stopping feature in the form of HOP --FF, cf. LSDJ's HFF (which is STOP in LGPT). While the same functionality may be accomplished using STOP in live mode, this change allows channel-stopping in song mode, which comes in handy when for instance a song is fully sequenced and a count-in or head is to be played only once at the start.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Use the HOP command with argument aaFF and note that it behaves similarly to STOP except only the current channel stops playing.

Test Configuration:

• Hardware: x64, aarch64 Linux
• Test steps:
  See above.

Checklist:

• I have performed a self-review of my code
• I have commented particularly in hard-to-understand areas
• I have updated CHANGELOG
• I have updated docs/wiki/What-is-LittlePiggyTracker.md reflecting my changes
• I have version bumped in sources/Application/Model/Project.h
• My changes generate no new warnings (build without your change then apply your change to check this)
• No LLMs or other sources or tools that may call this project's integrity or license into question have been used or consulted